At present three aprotic electrolytes are employed for aluminum electroplating, in the form of complex salt melts or organic solutions. The aluminum-yielding compounds of these respective electrolytes are:
1. Aluminum chloride AlCl.sub.3 and AlBr.sub.3, PA1 2. Al (III) hydride AlH.sub.3 in mixture with AlCl.sub.3, PA1 3. Al (III) alkyl, particularly Al(C.sub.2 H.sub.5).sub.3.
These form electrically conductive complex compounds with alkali metal halogens or hydrides. The baths which comprise these electrolytes must be protected from moisture and oxygen since, for example, in the case of the third type of electrolyte listed above, one mole of O.sub.2 effects the destruction of 2 mols of electrolyte: EQU 2Al(C.sub.2 H.sub.5).sub.3 +O.sub.2 .fwdarw.2(C.sub.2 H.sub.5).sub.2 -Al-O-C.sub.2 H.sub.5.
One mol of H.sub.2 O can make 2 to 3 mols of electrolyte ineffective because the aluminum ethylates are rendered incapable of building complexes, with the result that they show no electrical conductivity and their formation constitutes a loss of electrolyte.
With respect to the inert gas space that is maintained over the electrolyte in a cell of the type here under consideration, the electrolyte can be regarded as acting like a getter pump for O.sub.2 and H.sub.2 O. This getter pump tries to maintain unmeasureably small partial pressures of O.sub.2 and H.sub.2 O in its inert gas space, and to the extent that those substances are present in that space it consumes itself in removing them. Therefore, apparatus that is to be technically useful for electrolytic deposition of aluminum must exhibit a leakage rate for atmospheric gases which is as small as that of a high vacuum installation, in order for the electrolytes to have a useful life (about one year) that is economically satisfactory and to ensure that the amount of derivative product of the electrolyte that is deposited on the walls and service elements in the inert gas space is not excessive from the standpoint of machine technology.
For introducing and removing the workpieces that are to be electroplated, the electrolyte chamber must have at least one opening that can be sealed shut, and conventionally the workpieces are moved into and out of the electrolyte chamber through a lock which is intended to prevent oxygen and atmospheric moisture from entering the electrolyte. Because of the relatively large frame needed for supporting the parts that are to be electroplated, the provision of suitable mechanical vacuum valves would be very costly. Furthermore, it would be difficult to test the security of seal afforded by such valves while the apparatus was in operation, and any repair of such a valve would require shutdown of the electroplating cell.
In other applications and processes, generally similar problems have been successfully solved with the employment of U-shaped liquid locks of the type illustrated in FIG. 1. Adjacent to the electroplating vessel proper, which is designated by G, there is a lock S that is filled with liquid. The lock chamber S is communicated with the electroplating vessel G through an inverted-U-shaped passage that is filled with inert gas. The directions taken by workpieces moved into the electroplating vessel G are designated by arrows, and the pieces are removed by transporting them in the reverse direction. Inert gas is filled into the inert gas space I through an overpressure valve V which is communicated with that space, to provide for maintenance therein of a pressure slightly above atmospheric.
As might have been expected, an attempt to use such an arrangement in connection with an aluminum electroplating cell showed it to be wholly unsatisfactory. With an electrolyte system comprising the second of the above-given aluminum-yielding compounds, diethyl ether was tried as the lock chamber fluid; and toluol was tried with the third of the above given compounds. Other liquids were investigated for the purpose, but it was not possible to find one suitable for an aluminum plating process that provided an adequate barrier to the passage of oxygen and moisture between the atmosphere and the electrolyte space.
The transport of moisture through the lock fluid was found to be particularly high. This was apparently because the liquid in the lock chamber is agitated, rather than still, during locking in and out of the pieces to be plated, and because of the difference between temperatures outside the apparatus and inside it. By way of example, an experimental installation had in its lock 160 liters of toluol with a surface area of 0.25 m.sup.2, and had an electrolyte bath with 0.2 m.sup.2 of surface area, containing 80 liters of electrolyte, corresponding to about 270 mols of Al(C.sub.2 H.sub.5).sub.3. With a relative humidity of 40% to 50%, about 10 mols of H.sub.2 O passed through the lock daily, and therefore the electrolyte became unusable in much too short a time for economical operation. With this same installation the oxygen transport was found to be about 0.2 mols per day, and this value was likewise too high for an aluminum electroplating cell to be regarded as functioning and operating economically.
The transport of H.sub.2 O could be improved with the employment of known technical processes for drying toluol with the use of silica gel posts that are regenerated with freshly distilled dry toluol. This process is very expensive if the toluol is to be dried to about 5 to 10 ppm of H.sub.2 O from the moisture values of 220 to 280 ppm, which is specified for an atmosphere of 40% to 50% relative humidity, and with a very efficient drying circuit for the toluol to accommodate the speed with which moisture is taken up by toluol exposed to the atmosphere. For example, in the above-discussed investigation the water content of 200 ml of toluol, with a surface of 16 cm.sup.2, agitated by stirring, and with an air flow of 0.8 liters/min. (41% relative humidity) rose from 5 ppm to 190 ppm in one hour. A suitable circuit for drying toluol from such high moisture content values would be an almost economically prohibitive burden for an aluminum electroplating process, due to its technical costliness and its high energy consumption.